1. Field of the Invention
The present invention relates in general to the field of electronics, and more specifically to a system and method for isolating secondary transformer winding current during auxiliary power supply generation with an auxiliary-winding of the transformer.
2. Description of the Related Art
Many electronic systems include circuits, such as switching power converters to provide efficient power conversion from a voltage supply into a regulated output voltage. Often, a controller controls the power conversion process of a switching power converter. The switching power converter converts input power from a supply voltage source into an amount of output power utilized by a load. The controller utilizes a supply voltage derived from the same supply voltage being converted by the switching power converter.
FIG. 1 depicts a flyback-type switching power converter 100 that converts the input voltage VIN into a constant current iS and load voltage VLD on the side of the secondary-winding 116 of the transformer 112 and to a converter supply voltage VDD on the side of the auxiliary-winding 124. In at least one embodiment, the input voltage VIN is a rectified nominally 60 Hz/110 V line voltage in the United States of America or a nominally 50 Hz/220 V line voltage in Europe and the People's Republic of China. The controller 102 generates a switch control signal CNTRL to control the flyback-type, switching power converter 104. The control signal CNTRL controls the conductivity of field effect transistor (FET) switch 106 to control the primary current iP to meet the power demands of load 108. For an n-channel FET, the FET conducts (i.e. ON) when the switch control signal CNTRL is a logical one and is nonconductive (i.e. OFF) when the switch control signal CNTRL is a logical zero.
When the FET 106 conducts, the primary current iP ramps up through the primary winding 110 of transformer 112. The dot convention of transformer 112 and the diode 114 prevent flow of the secondary current iS from the secondary-winding 116 when FET 106 conducts and the primary current iP is flowing into the primary winding 110. When the controller 102 generates the switch control signal CNTRL to turn FET 106 OFF, the primary current iP falls to 0, and the voltage across the primary winding 110 reverses (also referred to as “flyback”). During the flyback, the secondary current iS quickly rises and charges capacitor 118. Capacitor 118 provides an output voltage VLD and current to the load 108. The load can be any type of load including one or more light emitting diodes. A diode and resistor-capacitor filter circuit 120 provides a path for voltage perturbations.
After the switching power converter 104 begins operation, an auxiliary power supply 122 provides the supply voltage VDD for controller 102. The auxiliary power supply 122 includes an auxiliary-winding 124 with the same dot convention as the secondary-winding 116. The FET 126 is biased by a fixed gate voltage VG to conduct the auxiliary current iAUX through diode 130 and resistor 132 to the VDD voltage node. When the controller supply voltage VDD falls below the gate voltage VG minus a threshold voltage VTH of the FET 126, the FET 126 conducts and charges the VDD node, which charges capacitor 128. When the voltage VDD reaches VG+VTH, the FET 126 stops conducting. Capacitor 128 stores energy to assist in providing a relatively constant value of the controller supply voltage VDD.
The controller supply voltage VDD varies in accordance with varying power demands by controller 102. Thus, the auxiliary power supply 126 provides power to the controller 102 in accordance with the varying power demands of controller 102. When the auxiliary power supply 126 provides charge to the capacitor 128, the auxiliary power supply 126 takes charge from the primary winding 110 that would otherwise be provided to the secondary-winding 116. Since the power demands of the auxiliary power supply 122 are not monitored, the amount of power actually delivered to the secondary-winding 116 and, thus, the load 108 is not accurately known.